This invention relates to a broadband, digitally controlled impedance network capable of producing a multitude of complex reflection and transmission coefficients for use in characterization of non-linear power or low noise linear transistors. Both power and noise characterization of the transistors requires measurement to be done under various loading conditions.
Engineers traditionally require noise-figure and gain circle data from an active device when a two-port network such as a low noise amplifier (where the information is used to optimize gain and noise figure) is designed. In a digital communication system, excessive noise interferes with a receiver's ability to differentiate between high (a digital "1") and low (a digital "0") level data bit. Excessive noise raises the system's bit-error rate and this results in lost information. In an analog radar system, for example, noise degrades the receiver's overall sensitivity. Excessive noise limits the radar receiver's ability to extract signal information from the system noise floor, and results in ambiguous returns and undetected threats.
Microwave load dependent devices and circuits can be accomplished by terminating the device in a mechanically or electromechanically controlled network consisting of continuously variable tuners, attenuators and phase shifters. Measurements must also be made at various load conditions to properly characterize these low noise or power devices. The measurements referred to above are accomplished by inserting a device called a tuner on the input and output of a transistor. The input tuner is adjusted for minimum noise figure and the output tuner for maximum available gain at minimum noise figure. These measurements are referred to as "Source-pull" measurements.
The noise figure of the transistor as a function of source admittance is given by the following formula: EQU F=Fmin+(Rn/Gs).vertline.Ys-Yopt.vertline..sup.2
where F is the device noise factor as a function of source admittance, Fmin is the minimum noise figure from the device under test (DUT), and Yopt is the complex source for minimum noise figure. Rn is the equivalent noise resistance or the parameter which defines the sensitivity of noise figure to changes in the Ys, the source admittance. In the foregoing equation, Gs is the source conductance. Yopt is itself made up of two scalar values, as follows:
Yopt=Gopt+jBopt
Gopt=Optimum source conductance
Bopt=Optimum source susceptance
Gain parameters calculated are based on the following formula: EQU 1/G=1/Gmax+(Rg/Gs).vertline.Ys-Yopt.vertline..sup.2
where Gmax is the maximum available gain achievable from the DUT, Yopt is the complex source admittance for maximum available gain, Gmax, and Rg is the equivalent gain resistance or the parameter which defines the sensitivity of available gain to changes in source admittance.
The traditional prior art means of varying source impedance is with a mechanical tuner. Unfortunately, mechanical tuners do not offer long-term stability, since they must be adjusted manually before each measurement and thus are not suitable for automatic testing use.
The basic theories under which noise measurement are assessed and descriptions of operative devices are given in: Adamian, V., "Stable Source Aids Automated Noise-Parameter Measurements, "MSN+CT, February 1988, pp. 51-58 and Froelich, R. K., Automated Tuning for Noise Parameter Measurements Using a Microwave Probe, Microwave Journal, March 1989, pp. 83.96.
One example of a programmable, solid state two port microwave network is disclosed in U.S. Pat. No. 4,502,028. This patent discloses a tuner using a plurality PIN diodes positioned behind a 3 dB Coupler, two 4.7 dB Couplers and a fixed Phase Shifter set for 45 degrees at the center of the operating frequency range. The impedance states are selected by turning on the PIN diodes in various combinations. As disclosed in the referenced patent, PIN diodes have low capacitance and very high impedance when reverse biased and can also withstand large RF voltages. In the above-referenced patent, a PIN diode effectively acts as an open circuit or a short circuit to an RF signal depending on the biasing of the diode. The device disclosed by the above patent suffers from the small number of load conditions (limited to the number of diodes to the power of two) a limited frequency range (one octave or less) and limited maximum reflection magnitude (0.707 maximum due to coupling networks). Providing an electronically controllable impedance network with a large number of load conditions, a broad frequency range, and maximum reflection greater than prior art devices would enhance the ability to automate the test process and increase the speed of measurement.
Mechanically controlled networks afford wide frequency range but suffer from slow transition time from one state to another. Such mechanically controlled tuners use stepper motors to move a metallic probe in a slab line structure to create the Voltage Standing Wave Ratio (VSWR) discontinuity. Mechanical systems wear, repeatability is a problem, and their speed will not compete with solid state tuner devices.